1. Field of the Invention
The present invention relates to pistons for internal combustion engines. In particular, the present invention relates to a piston having configuration that achieves flexible support of the skirt sections of the piston while minimizing stresses in a transition area between the piston bosses and the piston head.
2. Description of Related Art
In an internal combustion engine, each cylinder has piston slidably received therein. The piston is operatively connected to a crankshaft in a crankcase through a connecting rod. The connecting rod is typically connected to the piston by a piston pin. The piston pin is received within a pair of piston pin bosses that are either formed in or connected to the piston. Typically, pistons are formed with skirt sections on opposing sides of the piston pin bosses. The outer surfaces of the skirt sections serve to stabilize the piston within the cylinder during operation. The outer surfaces of the skirt sections confront the cylinder wall during the combustion cycle to take up side loads imparted on the piston in order to keep the piston aligned with the cylinder during operation.
The skirt sections are typically joined to each other and the piston pin bosses by connecting walls. The connecting walls serve to support the skirt sections and connect the piston pin bosses to the underside of the piston head. In order to achieve flexible or variable or soft support for the skirt sections such that the skirt sections maintain sufficient contact with the sides of the combustion chamber, it is desirable to locate the connecting walls as far apart as possible. On the other hand, in order to minimize stress in the transition area between the piston boss and the underside of the piston head, it is advantageous to have the smallest possible distance between the connecting walls. In the pistons currently known in the art, only one of these conditions can be optimized. The profile of the piston skirt in the vertical direction of reciprocation is typically flat with the slightly tapered ends to provide smooth guidance of the skirt up and down in the cylinder.
For example, DE 196 43 778 C2 discloses a light weight piston. The piston includes a pair of skirt sections that are located on opposing sides of a pair of piston pin bosses. The skirt sections are connected together by spatially curved connecting walls. The connecting walls are convexly curved in the direction of an outer side of the piston. These connecting walls, however, do not optimally support the piston pin bosses. This arrangement, also, causes major stresses in the transition area between the piston pin bosses and the underside of the piston head.
EP 0 835 390 A1 discloses another light alloy piston. The piston includes supporting skirt sections that are joined to each other by connecting walls. The skirt sections and the connecting walls are arranged in the shape of an H. The connecting walls extend in an area of the inner lateral faces of the piston pin bosses and are concave in the direction of the outer side of the piston. With this construction, the piston pin bosses are well connected to the underside of the piston head to minimize stress. This construction, however, produces a rather rigid or stiff support for the skirt sections. This produces piston noise.
DE 34 25 965 A1 also discloses a light alloy piston having flat connecting walls between the skirt sections are flat. With this arrangement, it is possible to provide flexible or variable support for the skirt sections. The piston pin bosses, however, are not joined to the underside of the piston head in an optimal manner. As such, major stresses can occur in the transition area between the piston pin bosses and the underside of the piston head, which may lead to cracking in extreme operating conditions.
U.S. Pat. No. 4,989,559 to Fletcher-Jones discloses a piston for an internal combustion engine. The piston pin bosses of the piston are supported by a pair of planar webs and a plurality of support ribs.
GB 2 238 596 A describes a piston with pin bosses and skirt sections that are joined to each other by connecting walls. The connecting walls are arranged in the area of the outer lateral face of the pin bosses, and are shaped so as to curve convexly outward. This connection to the underside of the piston head introduces major stresses.
An Ω piston having piston-pin bosses and skirt sections that are joined to each other by connecting walls is described in Innovating Piston for High Performance 4 Stroke Engine, drawing and development, by U. Panzeri, Gilardoni Vittorio S. P. A., 2nd International Seminar “High Performance Spark Ignition Engines for Passenger Cars,” 23rd to 24th November, 1995, Milano, Italy. When viewed in plan view, the skirt sections and the connecting walls take the shape of an Ω. This arrangement achieves even and precise clearance between the piston and the cylinder. The connecting walls are arranged near the inner lateral face of the piston pin bosses. Each connecting wall is curved in an S-shape between the area of the piston pin bosses and the skirt sections. With the Ω-piston arrangement, although the support for the skirt sections is relatively flexible, the stress distribution in the transition area between the piston boss and the underside of the piston head is not favorable.
WO 00/72116 discloses a method of producing a box piston. The connecting walls are arranged adjacent an inner surface of each of the piston pin bosses. The lower free ends of each of the connecting walls curve in one direction away from a center plane of the box piston.
EP 0838 587 A1 discloses a piston having a pair of skirt sections with concavely tapered edges when viewed from a plane of symmetry bisecting the piston. The connecting walls are following the curvature of the tapered edges of the skirt sections.